Mass flowmeter of the coriolis type

ABSTRACT

A mass flowmeter of the Coriolis type with a tube through which a medium flows during operation and with excitation elements for causing the entire tube or part thereof to perform a rotational vibration about a primary axis of rotation during operation. The excitation elements are electromagnetic and do not make contact with the tube during operation and have no components that are fastened to the tube. More The tube is made of an electrically conducting material, and the excitation elements include first members for causing an electric current to flow through the tube during operation and second elements for generating a magnetic field at the area of a portion of the tube. The magnetic field is perpendicular to the direction of the current and in operation either the current or the magnetic field changes its sign periodically.

FIELD OF THE INVENTION

The invention relates to a mass flowmeter of the Coriolis type with asensing tube through which a medium flows during operation and withexcitation means for causing the entire tube or part thereof tooscillate about an excitation axis of rotation during operation.

BACKGROUND OF THE INVENTION

Such a mass flowmeter is known from U.S. Pat. No. 4,658,657.

The known mass flowmeter comprises a loop-shaped tube (half a turn) thatforms a transverse branch at one side and two lateral branches clampedin at the opposite side in a mounting beam. The latter is mounted in asupport such that it can rotate about a central axis lying in the planeof the loop. An electromagnetic excitation system cooperating with themounting beam provides an oscillatory rotation (rotational vibration) ofthe mounting beam with the loop about the central axis. (The term‘excitation’ is here understood to mean ‘causing to oscillate’).

When a medium (gas or liquid) flows through the loop that rotates aboutthe central axis, Coriolis forces are generated in the transversebranch, resulting in an oscillation of the loop about a secondary axis.This oscillation, which is proportional to the flow, is superimposed onthe fundamental oscillation and leads to a phase shift between theoscillations performed by the ends of the transverse branch. The phasedifference is proportional to the Coriolis force and accordingly to theflow.

It is a disadvantage of the known system, however, that the mountingbeam used for the excitation of the loop constitutes an additional mass.This prevents a change in the excitation frequency as a function of thedensity of the medium flowing through the tube, with the result that ameasurement of the density (an additional property of a Coriolisflowmeter) becomes less accurate.

SUMMARY OF THE INVENTION

The invention has for its object inter alia to provide a flowmeter withan excitation system that is capable of measuring the density moreaccurately.

The mass flowmeter of the kind mentioned in the opening paragraph is forthis purpose characterized in that the excitation means are(electro)magnetic and in operation do not make contact with the tube andhave no components that are fastened to the tube. In other words: theexcitation means are free from the movable portion of the tube both inthe idle and in the operational state. It is noted that the tube may be,for example, a straight tube, a looped tube forming half a turn, alooped tube forming a full turn, or a looped tube forming a double turn.

The mass flowmeter according to the invention has an enhancedsensitivity because the (movable portion of the) tube is free fromadditional excitation components (=no added mass). A possibility ofcausing the tube to rotate is found in the use of a tube of magnetizablemetal material, such as soft iron, in combination with one or twoelectromagnetic coils that can be energized in a pulsating mode (i.e.utilizing the natural magnetism of the tube material: operating in theway of a relay).

An embodiment of the flowmeter according to the invention is for thispurpose characterized in that the tube is manufactured from amagnetizable material, and in that in operation the excitation meansgenerate a magnetic field configuration at the area of at least aportion of the tube that causes the tube to enter a swing excitationmode or a twist excitation mode.

A preferred embodiment, however, is characterized in that the tube ismanufactured from an electrically well conducting material, and theexcitation means comprise first means for causing an electric current toflow through the tube wall during operation and second means forgenerating a magnetic field transverse to the direction of the currentat the area of a portion of the tube, either the current or the magneticfield changing its sign periodically during operation, such that the(Lorentz) force generated by the product of the current and the magneticfield acts on the tube portion in a periodically changing direction. Inthis manner an oscillatory rotation (vibration) of the entire tube orpart thereof about the excitation axis (primary axis) can be generated.The advantage of the use of Lorentz forces is that they can be generatedin a simple manner by means of an electric current through the tube anda magnet, while generating sufficiently great forces at limitedconstructional dimensions for bringing the tube into oscillation.

The flowmeter according to the invention may be fitted with first(excitation) means that generate a direct current in the tube wall andwith second (excitation) means that generate a magnetic field with aperiodically changing sign.

However, an embodiment that can be very readily implemented ischaracterized in that the second means generate a permanent magneticfield and the first means generate an alternating current in the tubewall.

An embodiment for exciting the tube in the swing excitation mode ischaracterized in that the second excitation means comprise two magnetpoles situated opposite one another, between which poles an air gap ispresent, through which air gap a portion of the tube is passed, analternating current flowing through this tube portion in operation suchthat the tube portion in operation is subjected to an excitation byforce, resulting in an oscillatory rotation of the tube or tube portionabout the excitation (or primary) axis of rotation.

The above is applicable to a loop-shaped as well as to a straight tube.

An embodiment for exciting the tube in a twist mode is characterized inthat the excitation means provide two oppositely directed permanentmagnetic fields located at some distance from one another, each formedbetween two magnet poles located opposite one another between which anair gap is present, through which air gaps a portion of the tube ispassed, an alternating current flowing through said tube portion inoperation such that a torque excitation of the tube portion arises inoperation, resulting in a rotational vibration of the tube or tubeportion about an excitation axis of rotation.

Again, the above is applicable to a loop-shaped as well as to a straighttube.

The torque excitation described above may be achieved by means of twoseparate magnet yokes each provided with an air gap. A difficulty is,however, to ensure that the magnetic fields in the gaps are equallystrong. A preferred embodiment of the invention in this respect ischaracterized in that the second excitation means comprise acircumferential permanently magnetic magnet yoke that is arrangedparallel to a plane through the tube with two pairs of magnet polesarranged two by two in mutual opposition, between which pairs a firstand a second gap are present, in which gaps oppositely directed magneticfields are generated and through which gaps a portion of the tubeextends, an alternating current flowing through said tube portion inoperation such that a torque excitation of the tube or tube portionarises in operation, resulting in and oscillatory rotation of the tubeabout the excitation axis of rotation.

Again, the above is applicable to a loop-shaped as well as to a straighttube.

An alternative embodiment may be used for the generation of a constantor alternating magnetic field in the gap or gaps, which is characterizedin that the magnetic field is generated by means of an electric coilwound around a magnetic yoke with at least one gap, which coil isconnected to an electric circuit designed to pass a direct current or analternating current through said coil in operation, whereupon the firstmeans cause an alternating current or a direct current, respectively, toflow through the tube wall. This at the same time offers the possibilityto adjust the strength of the magnetic field.

The first means for generating a current in the tube wall may be meansthat inject a current directly into the tube wall, for example throughconnection terminals. Direct current injection, however, is a methodthat is less desirable for certain applications. Indirect currentinjection is preferred.

In this connection, an embodiment is characterized in that at least onetransformer core is provided around the tube, the tube constituting a(closed) secondary winding while a coil provided on the transformer coreconstitutes a primary winding, so that a current is induced in the tubewall when the primary winding is energized.

U.S. Pat. No. 4,658,657 mentions that two measuring devices are usedadjacent the ends of the transverse arm of the loop for measuring thephase shift between the oscillations performed by the ends of thetransverse arm. It is found that it is not possible to measure with ahigh accuracy with such an arrangement.

An embodiment of the invention provides a system with which a moresensitive measurement becomes possible.

The mass flowmeter of the kind mentioned in the opening paragraph is forthis purpose characterized in that it is provided with at least twooptical sensors for measuring deformation of the tube occurring underthe influence of a medium flowing through the tube, which sensors aresituated on either side of the point of intersection (pole) of theprimary axis of rotation and a tube portion of which the deformation isto be measured, the distance from each sensor to said point ofintersection being between 5% and 25% of half the length of said tubeportion.

Preferably, the optical sensors are opto-electronic sensors which eachcomprise a light source located at one side of the tube portion and aphotosensitive cell located at the opposite side of the tube portion inthe light path of the light source, such that the portion of the lightnot intercepted by the tube is measured.

An alternative embodiment is characterized in that the optical sensorsare opto-electronic sensors which each comprise a light source locatedat one side of the tube portion and a photosensitive cell located at thesame side of the tube portion in the path of the light reflected by thetube portion, such that either the intensity of the reflected lightincident on the photosensitive cell, or the location of the reflectedlight on the photosensitive cell, is measured.

Advantageous, furthermore, is a positioning of the light source relativeto the tube portion such that 40 to 60% of the active surface area ofthe photosensitive cell is illuminated by the light source in the idlemode.

The flowmeter according to the invention has an enhanced sensitivitythanks to the placement of the sensors close to the axis, because nowthe measured amplitude of the fundamental oscillation does not becometoo great in relation to that of the Coriolis force, as is indeed thecase in the arrangement of U.S. Pat. No. 4,658,657.

The sensitivity of the mass flowmeter according to the invention is evenfurther increased if a looped tube is used which is flexibly suspendedby means of its inlet and outlet tubes. An embodiment is for thispurpose characterized in that the loop follows a substantiallycircumferential, mechanically closed path, in that the loop is connectedto a flexible inlet tube and to a flexible outlet tube for the flowingmedium, and in that the loop is flexibly suspended by means of saidflexible inlet and outlet tubes such that the resulting suspensionallows a movement about two mutually perpendicular axes in the plane ofthe loop, one for the excitation movement and one for the Coriolismovement that arises when a medium is flowing through the tube.

BRIEF DESCRIPTION OF THE DRAWINGS

A few embodiments of the invention will be explained in more detail withreference to the drawing.

FIG. 1 is a perspective view of a Coriolis flowmeter according to theinvention with a U-shaped tube;

FIG. 2 shows the U-shaped tube of the flowmeter of FIG. 1 into which anelectric current is directly injected;

FIG. 3 shows the induction of a current in a tube portion by means of atransformer core with a coil;

FIG. 4A shows a permanently magnetic magnet yoke with one gap throughwhich a tube portion of a U-shaped tube extends;

FIG. 4B shows a detail from FIG. 4A;

FIG. 5 shows how a torque excitation is applied to a tube portion bymeans of two independently located permanently magnetic magnet yokes;

FIG. 6A is a perspective view and FIG. 6B a front elevation of apermanently magnetic magnet yoke with two gaps through which a tubeportion extends, showing how a torque excitation is applied to the tubeportion;

FIG. 7 shows a magnet yoke with a coil wound thereon and two gapsthrough which a tube portion of a U-shaped tube extends;

FIGS. 8 and 9 show alternative embodiments of a magnet yoke with gapsthrough which a tube portion of a U-shaped tube extends;

FIGS. 10A and B diagrammatically show an optical sensor operating withtransmitted light as used in the flowmeter of FIG. 1;

FIG. 11 diagrammatically shows an alternative arrangement with anoptical sensor operating with reflected light;

FIG. 12 shows an arrangement of two optical sensors and a tube portion;

FIG. 13A shows a phase difference between the signals of the sensors ofFIG. 12 measured when there is no flow through the sensing tube, andFIG. 13B shows the situation in which there is a mass flow through thesensing tube;

FIG. 14 is a block diagram representing the excitation, measurement, andprocessing of the measured values when a flowmeter according to theinvention is used;

FIG. 15 shows a looped tube that is flexibly suspended by its inlet andoutlet tubes;

FIG. 16 diagrammatically shows a Coriolis flow sensor arrangement havinga looped tube equipped with a torque excitation yoke and with Corioliseffect sensing means; and

FIG. 17 shows in greater detail the torque excitation yoke used in thearrangement of FIG. 16.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an embodiment of a Coriolis flowmeter 1 according to theinvention. It is provided with a frame having a base plate 2 whichsupports a tube 3 through which a medium flows during operation. Thetube 3 is a looped tube in this case constituting a half-turn, but itmay alternatively be, for example, a straight tube or a looped tube witha full turn (closed loop). Looped tubes are preferred because they aremore flexible than straight tubes. The tube 3 is fastened to the baseplate 2 by fastening means 4, 5. The means 4, 5 form clamping locationsrelative to which the tube 3 is capable of moving. The tube 3, which maybe made, for example, of stainless steel with a wall thickness ofapproximately 0.1 mm and a diameter of approximately 0.7 mm, forms avery lightweight construction within the scope of the invention whichcan be brought into resonance with a small amount of energy. Theexternal diameter of the tube will generally be smaller than 1 mm andits wall thickness 0.2 mm or less, depending on the external dimensionsof the loop 3 and the pressure the tube should be able to withstand (forexample 100 bar).

To realize a very light construction, no further components that givethe tube 3 an additional mass have been mounted to the tube 3. This ispossible because Lorentz forces are used for exciting the tube, i.e.causing it to vibrate, in the construction of FIG. 1 (Lorentz force:electrons moving in a magnetic field undergo a force having a directionperpendicular to both the direction of the magnetic field and thedirection of the current). These forces are generated in the flowmeter 1of FIG. 1 because an electric current is passed through the wall of the(electrically conducting) tube 3 while at the same time a permanentlymagnetic magnet yoke 6, 7, 8, 12 (12 representing a permanent magnetwith one pole directed towards yoke portion 6 and an opposite poletowards yoke portion 7) provided with a central opening generates twooppositely directed magnetic fields in the plane of the tube 3transversely to the direction of the current. An electric current I canbe directly injected into a (U-shaped) tube 13 of electricallyconducting material in that, as shown in FIG. 2, a current source 14, anAC source in this case, is connected to ends 15, 16 of the tube 13 viaconnection terminals 17, 18.

Preferably, however, an electric current is generated in the tube bymeans of induction. FIG. 3 shows how this is achieved for the sameU-shaped tube 13 as in FIG. 2. A tube portion 21 of the U-shaped tube 13here extends through a transformer core 22. A primary coil 23 is woundon this core 22, which coil can be energized by a current source 24connected thereto. The tube portion 21 thus acts as a secondary coil inwhich a current I is induced when a current flows through the primarycoil 23. The tube portion 21 for this purpose forms part of anelectrically closed loop (indicated with a broken line). This loop maybe closed via the tube or via the housing. The transformer coil 22 withthe primary coil 23 is provided around an ‘external’ portion 21 of thetube 13 outside the fastening points 19, 20 and stationary duringoperation in this case. The tube portion 21 may serve, for example, asan inlet or outlet conduit. Alternatively, however, the transformer core22 may be arranged around an ‘internal’ portion of the tube that movesduring operation, lying within the fastening points 19, 20, providedthere is enough space. If there is little space, the one, comparativelybulky transformer core with primary coil may be replaced by two smallertransformer cores each with a primary coil, for example one around eachof the legs of the U-shaped tube 13.

FIG. 4A shows by way of example how the magnetic field required forgenerating the Lorentz forces can be obtained. A tube portion 25 of theU-shaped tube 26 is for this purpose passed through an air gap 27 of apermanently magnetic magnet yoke 28. The tube 26 is clamped in inlocations 33 a and 33 b. The yoke 28 comprises a core 29 of a softmagnetic material (for example soft iron) with the North pole N andSouth pole S of a permanent magnet 30 arranged in the path formed bysaid material such that the lines of force of the magnetic fieldgenerated in the air gap 27 extend parallel to the plane of the U-shapedtube 26 and are perpendicular to the current I, which is either directlyinjected into the tube 26 or induced in the tube 26. All this is shownin detail in FIG. 4B. This shows how the poles 31 and 32 of the magnetyoke 28 of FIG. 4A enclose the air gap 27, as well as the magnetic linesof force B of the magnetic field generated in the air gap 27. The resultis that a (Lorentz) force F is generated under the influence of thecurrent I that passes through the magnetic field, causing the tubeportion 25 to move, for example, in forward direction (shown in brokenlines in FIG. 4A). When the current I flows in the opposite directionthrough the tube, a (Lorentz) force will be generated in the oppositedirection, i.e. moving the tube portion 25 to the rear. The excitationby force described here causes the tube to move about an excitation axisof rotation that passes through the clamping locations 33 a, 33 b.

FIG. 5 shows the use of two magnet yokes 34 a and 34 b with permanentmagnets 39 a, 39 b incorporated therein, conforming to the type 28described with reference to FIG. 4, which magnets are located at somedistance from one another and each have an air gap 35 a, 35 b, in whichgaps oppositely directed magnetic fields are generated. A U-shapedsensing tube 37 is clamped in in locations 38 a, 38 b. A tube portion 36of the U-shaped tube 37 extends through the two gaps 35 a, 35 b. When acurrent I flows through the tube 37, the generated Lorentz forces willcause the left-hand section of the tube portion 36 present in the gap 35a to move, for example, in forward direction and the right-hand sectionof the tube portion 36 present in the gap 35 b to move to the rear. Whenthe direction of the current I is reversed, the right-hand section ofthe tube portion will move forward and the left-hand section to the rear(shown in broken lines). The torque excitation described here causes thetube 37 to rotate about an axis of rotation S′ that coincides with themain axis of symmetry of the U-shaped tube 37. A problem with thisarrangement is, however, that it is difficult to provide the magnetyokes with magnets of exactly the same strength.

FIG. 6A is an elevation of an integrated permanent magnet yoke 40 withwhich this problem is solved. The integrated yoke 40 has a first pair ofmutually opposed magnet poles 41 a, 41 b and a second pair of mutuallyopposed magnet yokes 42 a, 42 b. A respective air gap 43, 44 is formedbetween the poles of each of the pairs. The tube portion 46 the U-shapedtube 47 extends through these gaps. A permanent magnet 45 is positionedin the path of the, circumferential, yoke 40 with its North and Southpoles oriented such that oppositely directed magnetic fields B and B′are generated in the air gaps 43, 44. Given a direction of the current Ias indicated in the diagrammatic FIG. 6B, which is a front elevation ofthe assembly of FIG. 6A, Lorentz forces F (directed to the rear) and F′(directed to the front) will now act on the tube portion 46, whichforces will be reversed when the current direction in the tube wall isreversed. This torque excitation causes the tube 47 to perform areciprocating rotational movement (vibration) about an axis 48 whichcoincides with the main axis of symmetry of the U-shaped sensing tube47. The permanently magnetic yoke 40 with the two air gaps for torqueexcitation is designed such that the values of the forces F and F′ areequal in principle and oppositely directed. A non-ideal torqueexcitation arises when the forces have different values. In the idealcase in which they are exactly the same, a pure torque or moment offorce arises equal to the product of the force F and the spacing betweenF and F′. The direction of the torque vector (usually denoted T) then isalong the centerline 48 of the yoke 40 in FIG. 6B.

FIG. 7 shows an alternative embodiment of an integrated magnet yoke. Themagnet yoke 49 with a magnet core 51 provided with two air gaps 50 a, 50b is not energized by means of a permanent magnet in this case, but bymeans of a coil 52 wound on the magnet core 51 of the yoke 49 andconnected to a direct or alternating current source 53. A tube portionof a U-shaped tube extends through the air gaps 50 a, 50 b.

FIG. 8 shows a magnet yoke 54 where the excitation of the U-shaped tube55 takes place in the bends of the U-shaped tube 55 extending throughthe air gaps 56 a, 56 b. The yoke 54 is energized by a permanent magnet57, which is shown in the center of the upper leg 58 of the yoke 54 inthe construction of FIG. 8, but which may be accommodated in analternative location in the yoke. This is also true for the yokes shownin the other Figures.

FIG. 9 shows a magnet yoke 59 where the excitation of the U-shaped tube60 takes place on the lateral arms 62, 63, i.e. even below the bends 64,65, of the U-shaped tube 60 extending through the air gaps 61 a, 61 b.

It will be obvious that all aspects of the invention explained withreference to U-shaped tubes are equally valid for other shapes of tubesthat can be used in Coriolis flowmeters, i.e. not just tubes having ahalf-open turn, but also straight tubes and tubes forming a closed, fullturn.

Particularly suitable for use with the excitation principle set outabove is a loop-shaped tube that forms a mechanically closed rectangularturn, wherein the start and end points of the turn are connected to acentral inlet tube and a central outlet tube, respectively, theloop-shaped tube being resiliently suspended by means of said inlet andoutlet tubes (FIG. 15).

The basic idea of the invention is to achieve excitation withoutadditional components having to be provided on the tube for thispurpose. This is possible because properties of the tube itself areutilized. The excitation may be achieved not only by means of Lorentzforces, but also by utilizing the magnetic properties of the tubeitself. In that case a tube of magnetizable material is used incombination with one or two coils that are driven in a pulsating mode,generating a magnetic field so as to magnetize the material of the tubelocally.

The inventive principle may also be used in a Coriolis flowmeter havinga double tube, both of a type in which the directions of flow of themedium are mutually opposed and of a type in which the directions offlow of the medium are the same.

The full benefit of the inventive idea, however, can only be obtained ifthe detection of the deviations of the tube under the influence of aflowing medium (the detection of the Coriolis effect) takes place suchthat no additional components need be mounted to the tube.

For this purpose, according to the invention, one or several opticalsensors are used, referenced 11 a, 11 b, and 11 c in FIG. 1. The opticalsensors in the construction of FIG. 1 are arranged in the centralopening of the magnet yoke 6, 7, 8, 12 such that they can interact withthe tube. A temperature sensor is referenced 14.

FIG. 10A shows one of the optical sensors, in this case opto-electronicsensor 11 a, in more detail. It comprises a U-shaped housing 68 with alight source 66 (for example a LED) at the inner side of one leg of theU, and a light-measuring cell 67 (for example a phototransistor) at theinner side of the other leg of the U. The opto-electronic sensor 11 a isarranged such that a tube portion 69 can move between the legs of theU-shaped housing 68. During operation, the tube will cover the lightpath between the light source 66 and the photocell 67 to a greater orlesser degree.

FIG. 10B shows in greater detail how the tube portion 69, owing to itsmovement, blocks out a larger or smaller portion of the light beam 71transmitted from the light source 66 to the photocell 67. The photocell67 produces a signal u (V) that can be measured by a meter. The lightbeam may be a parallel beam or a divergent beam.

FIG. 11 shows an alternative to the sensor arrangement of FIG. 10. Herethe tube portion 69 and the light source 70 are so arranged that thetransmitted light beam 71 is incident on a photocell 72 after beingreflected by the tube portion 69. When the tube portion performs areciprocating movement during operation, the reflected spot will moveover the surface of the photocell 72. The tube portion 69 may beprovided with a reflecting surface at the side of the photocell 72, ifso desired.

FIG. 12 diagrammatically shows the detection by means of twoopto-electronic sensors 11 a, 11 b. According to one aspect of theinvention, these are located on either side of, and preferablysymmetrically with respect to, the location where the axis of rotation,about which the excitation means cause the tube to rotate, intersectsthe tube portion 69. This point of intersection is denoted the rotationpole P. The sensors 11 a, 11 b are preferably at a short distance fromthis pole. Said distance should be sufficiently small for ensuring thatthe measured contribution of the excitation is of the same order ofmagnitude as the measured contribution of the Coriolis forces. Thesensors measure the (sinusoidal) displacements (in mm) of points of thetube as a function of time (in seconds) by means of a voltage.

FIG. 13A shows the output signals of the sensors 11 a, 11 b for the casein which no medium flows through the tube (zero flow), the curveindicated with arrow 1 representing the measuring signal of sensor 11 aand the curve indicated with arrow 2 representing the measuring signalof sensor 11 b. The phase difference is 180°.

FIG. 13B shows the situation in which in which a medium does flowthrough the tube. The phase difference now is smaller than 180°. If therotation pole does not lie exactly in the center between the first andthe second sensor, however, the result of the measurement will not beaccurate.

A more accurate measurement is possible if a third sensor is placedadjacent one of the sensors of FIG. 12 and in line with these sensors. Aphase difference between the sensors 11 a and 11 b owing to a possibleshift of the rotation pole can be corrected by means of the measuringsignal from the third sensor. Without flow this difference is 180° inthe case of a symmetrical sensor arrangement, while in the extreme casein which a sensor lies on the pole it is no more than 90°. The threesensors supply three measured values while there are also three unknownfactors, i.e. the two—different—phase angles of the first and the secondsensor and the position of the rotation pole between the first and thesecond sensor. The value measured by the third sensor can be used in aprocessing device for determining the location of the rotation pole,whereupon the—equal—phase angles of the first and the second sensor canbe determined for a fictitious pole position that does lie centrallybetween the first and the second sensor. The measuring and detectionsystem described here does not require any amplifier, so that noundesirable phase shifts are caused, and is suitable for use with allCoriolis flowmeters.

FIG. 14 is a block diagram showing the operation of an embodiment of aCoriolis flowmeter according to the invention. An electric current I isinduced in a Coriolis tube system 75 by means of two coils 73, 74 woundon two cores. The coils 73, 74 are energized by an amplifier A that iscontrolled from a digital signal processor 77 via an AD/DA converter 76.A magnetic field H transverse to the direction of the current I isapplied across the tube system 75. The tube system 75, or a portionthereof, starts to vibrate under the influence of H and 1. Superimposedon this vibration is a vibration caused by Coriolis forces when a mediumF flows through the tube system. The movements of the tube system aremeasured by sensors S1 and S2, or sensors S1, S2, and S3. The analogsignals from the sensors S1, S2, (S3) are supplied to an AD/DA converter76. The output signals of the AD/DA converter are supplied to a(digital) signal processor 77. The digital signal processor 77 generatesan output signal 0 that represents the mass flow.

FIG. 15 is a perspective view of a mechanically closed, loop-shaped tube78 (rectangular in this case), but in an alternative embodiment theclosed, loop-shaped tube may have, for example, a delta shape. A firstend 79 of the loop 78 is connected to a flexible inlet tube 80 whichsupplies a medium flow F, and a second end 81 of the loop 78 isconnected to a flexible outlet tube 80 which discharges the medium flowF. The looped tube 78 and the tubes 80, 81 are preferably bent from onepiece of tubing. The loop 78 comprises a first transverse tube portion84 which is connected to first ends of two lateral tube portions 85, 86.The latter are connected by their second ends to two second transversetube portions 87, 88 that each have approximately half the length of thefirst transverse tube 84. The inlet and outlet tubes 80, 82 in thisconstruction extend symmetrically through the center of the loop 78, lieclose together or against one another, and are mechanicallyinterconnected in locations referenced b, for example by means ofsoldering or welding. They are fastened, next to or against one another,in a recess 83 a of a fastening means 83 which in its turn is fastenedto a frame (not shown). The loop 78 is resiliently suspended from theframe of the flowmeter (not shown) by means of the inlet and outlettubes 80, 82 (and the fastening means 83). The looped tube 78 maycooperate with a permanently magnetic magnet yoke for the purpose ofexcitation, such as the magnet yoke described with reference to FIG. 1and comprising yoke portions 6 and 7 located opposite a lower yokeportion 8 with two air gaps 9 and 10 enclosed between them, a magnet 12being placed in the path of the magnet yoke. For example, the tubeportions 87, 88 may extend through the air gaps of the magnet yoke(magnet yoke around the upper transverse tubes). When an alternatingcurrent flows through the looped tube 78, the loop will perform anoscillatory rotation about an axis (the excitation axis) extending inthe plane of the looped tube under the influence of the Lorentz forcesgenerated in the air gaps of the yoke by the current and the oppositelydirected magnetic fields (so-termed torque excitation). When a mediumflows through the tube 78, Coriolis forces are generated which cause aCoriolis effect. The Coriolis forces cause the tube 78 to oscillateabout a Coriolis response axis that is perpendicular to the excitationaxis. Coriolis effect sensors may be arranged in the central opening ofthe magnet yoke (and accordingly cooperate with the upper transversetubes during operation).

An alternative is to arrange the sensors adjacent the lower transversetube portion 84 such that they can cooperate with the lower transversetube during operation. Depending on the location of the torqueexcitation yoke relative to the rectangular looped tube of FIG. 15, thetube can be made to enter either a swing mode or a twist mode. That is:either it is twisted about the central axis of symmetry between theinlet and outlet tubes, or it is made to swing about an asymmetricalexcitation axis transverse to the central axis of symmetry, in whichcase the torque excitation yoke cooperates with a lateral tube portion.

FIG. 16 diagrammatically shows a flowmeter of the Coriolis type with alooped sensing tube 90 of the kind shown in FIG. 15. The looped tube 90has two ends which are connected to an inlet tube 91 for a fluid mediumF and an outlet tube 92, respectively. The inlet and outlet tubes 91, 92are interconnected, as were the inlet and outlet tubes 80 and 82 of FIG.15, for example through soldering in locations b or spot welding, andthey are fixed in fastening means 94 in a location remote from theirconnections to the looped tube 90. The fastening means 94 shown herecomprise a block having a central recess in which the inlet and outlettubes are accommodated. The block has two openings for fastening to aframe by means of bolts. The tube 90 is excited in the swing mode inthis case. For this purpose, a magnet yoke 95 with two air gaps 100 and101 is placed at one of the lateral sides of the looped tube 90 suchthat the leg 93 a of the loop runs through the air gaps 100, 101. Theyoke has an upper part with two yoke portions 96, 97 between which apermanent magnet 98 is arranged with a South pole S directed towards theyoke portion 96 and a North pole N directed towards the yoke portion 97.Oppositely directed magnetic fields B and B′, which are of equalstrength in the ideal case, are generated in the air gaps 100, 101between the upper portion 96, 97, 98 and the lower portion 99 by thisconstruction. When an alternating electric current/flows through thetube 90, these fields B′ and B exert a torque excitation on the tubeportion 93 a. The tube 90 will perform a swinging movement about an axisof rotation (X) owing to the torque excitation when traversed by analternating current I. The excitation axis of rotation X in thisembodiment, therefore, is perpendicular to the inlet and outlet tubes.The yoke 95 is a torque generator.

An alternating current/is induced in the tube 90 in the same manner asin the embodiment of FIG. 3. For this purpose, the lateral portions 93a, 93 b of the tube 90 are passed through bores in the respectivetransformer cores 102 and 104 on which respective coils 103 and 105 havebeen wound at the sides that face one another. The invention, however,is not limited to this. For example, the transformer or coil cores maybe provided in alternative locations of the tube 90.

When a fluid F flows through the tube 90 oscillating about theexcitation axis of rotation X, a Coriolis force arises which causes aCoriolis effect. The Coriolis effect is measured with a Coriolis sensor.The Coriolis sensor used in the present embodiment is a system ofcontactless optical sensors 106 a, 106 b, 106 c identical to the systemof contactless optical sensors 11 a, 11 b, 11 c of the construction inFIG. 1, but the invention is not limited thereto.

Two of the optical sensors, 106 a and 106 b, are arranged symmetricallywith respect to the excitation axis of rotation (the axis of rotation Xin this case) in the construction of FIG. 16. The optical sensors 106 a,106 b (and 106 c) here cooperate with the lateral portion 93 b of theloop 90, which portion is located opposite the portion 93 a thatcooperates with the magnet yoke providing the torque excitation.

FIGS. 15 and 16 show a looped rectangular tube with a more or lesssquare circumference. This was found to be a favorable shape as regardsthe sensitivity, given the correct surface area. If this should befavorable for the placement of the excitation means, the currentinduction means, and/or the Coriolis effect sensing means, however, itis possible to make the loop, for example, narrower and proportionallyhigher.

The operation of the integrated magnet yoke 95 will now be explainedwith reference to FIG. 17. The placement of the permanent magnet 98between the upper yoke portions 96 and 97 generates oppositely directedmagnetic fields B and B′ of equal strength in the air gaps 100,101. If Bin the air gap 100 is directed towards the yoke portion 99 and thecurrent direction is as shown in FIG. 17, then a (Lorentz) force Fdirected to the front will act on the tube 90. At the same time, themagnetic field B′ in the air gap 101 is directed towards the yokeportion 96. This results, in combination with the current I, in a(Lorentz) force F′ on the tube 90 directed towards the rear. Accordinglythere is a torque excitation. The forces acting on the tube are reversedwhen the current I through the tube changes direction. The supply of analternating current to the tube 90 thus leads to a swinging movement ofthe looped tube 90 about the axis of rotation X.

In brief, the invention relates to a mass flowmeter of the Coriolis typewith a tube through which a medium flows in operation, and withexcitation means for causing the entire tube or a portion thereof toperform a rotational oscillation about an excitation axis of rotation inoperation, wherein the excitation means are electromagnetic, do not makecontact with the tube during operation, and have no components that arefastened to the tube.

The Coriolis effect sensors are preferably optical sensors, also do notmake contact with the tube, and do not comprise components that arefastened to the tube.

1. A mass flowmeter of the Coriolis type with a sensing tube throughwhich a medium flows during operation and with excitation means forcausing the entire tube or part thereof to oscillate about an excitationaxis of rotation 7 wherein the excitation means in operation do not makecontact with the tube and have no components that are fastened to thetube, wherein the excitation means generate two magnetic fields, eachacting on a portion of the tube, in order to achieve a Lorentz forcetorque excitation of the tube or tube portion about the excitation axis.2. A mass flowmeter of the Coriolis type with a sensing tube throughwhich a medium flows during operation and with excitation means forcausing the entire tube or part thereof to oscillate about an excitationaxis of rotation, wherein the excitation means in operation do not makecontact with the tube and have no components that are fastened to thetube, wherein the excitation means comprise a magnet yoke the sensingtube being U-shaped and the magnet yoke having two air gaps throughwhich opposite portions of the U-shaped tube extend, in order to achievea Lorentz force torque excitation of the tube or tube portion about theexcitation axis.
 3. A mass flow meter as claimed in claim 2, wherein themagnet yoke is energized by a permanent magnet accommodated in themagnet yoke.
 4. A mass flow meter as claimed in claim 3, wherein themagnet yoke has an upper leg remote from the air gaps and in that thepermanent magnet is accommodated in the center of the upper leg.
 5. Amass flow meter as claimed in claim 2, wherein the U-shaped tube has twolateral arms, and in that the excitation is on the lateral arms of theU-shaped tube.
 6. A mass flow meter as claimed in claim 2, wherein theU-shaped tube has two lateral arms, and in that the excitation is on thebends of the lateral arms of the U-shaped tube.
 7. A mass flowmeter ofthe Coriolis type with a sensing tube through which a medium flowsduring operation and with excitation means for causing the entire tubeor part thereof to oscillate about an excitation axis of rotation,wherein the excitation means in operation do not make contact with thetube and have no components that are fastened to the tube, wherein theexcitation means comprises a magnet yoke which is adapted to function asa torque generator for the sensing tube.